This invention relates to video special effects, and more particularly to a controllable noise pattern generator for use in video special effects.
In the prior art, random number generators or random pattern generators have been used in video special effects as video noise sources to modify a matte with a textured effect. These textured mattes can then be used to create a textured "wipe" signal for controlling a transition between two other video sources, or to produce a rough edge or texture on some other pattern.
A pseudo-random number generator (PRNG) can be used to produce a series of pseudo-random numbers that translate into a pattern of noise on the raster scan of a video display. Each number can be used directly as a luminance or chrominance value for individual pixels, or, preferably, each number can be used as a key or mixing control signal to randomly mix two other video signals on a pixel-by-pixel basis.
A source of pseudo-random numbers that can be used for this purpose is a linear feedback shift register (LFSR) with two or more stages exclusive-ORed (XORed) together and fed back to the input to create a pseudo-random pattern generator. The desired pseudo-random pattern output is obtained by combining the output from a number of stages into a parallel word that is then interpreted as a binary number.
While the sequence of outputs that is produced from a pseudo-random pattern generator has the property of being pseudo-random, the same pattern repeats identically every 2.sup.n -1 clock cycles, where "n" is the number of stages in the shift register. For example, a 10-stage linear feedback shift register with stages 7 and 10 exclusive-ORed together and fed back to the input to stage 1 will produce a pseudo-random output sequence that repeats every 2.sup.10 -1 clocks, with that same sequence then repeating again and again.
If the pseudo-random pattern generator is clocked at the frequency of video pixel occurrences and the numbers that it produces are used to create 10-bit digital video key signals and those key signals are used to control the mixture of two video sources, a "noise pattern" is produced in the resulting video image. If the length of the shift register produces a sequence of pseudo-random numbers that has fewer elements than the number of pixels in a television raster, this noise pattern repeats and becomes noticeable in direct proportion to how many times it is repeated in one field of video, i.e., the smaller the pattern is and the more times it is repeated on the screen, the more visible it becomes. Conversely, if the length of the shift register is chosen to produce a sequence of pseudo-random numbers larger or nearly as large as the number of pixels in a field of video, the observed pattern is no longer detectable by the human eye.